In portable radio receivers and especially in selective call receivers and related applications, size and weight are extremely important factors. In the past, many selective call receivers have used antenna systems that have tended to increase the size and weight of the receivers significantly. As compactness and light weight have become more important in the design of modern selective call receivers, compact and light weight antenna systems also have become more important.
The need for small size and weight is particularly strong in a selective call receiver designed to be carried on the wrist. However, a degradation in antenna system performance to achieve size and weight reduction is not desirable. Furthermore, a wrist carried selective call receiver is operated in many different positions. It may be held in front of the face, at the side of the human body, outstretched from the body, or placed on various conducting and non-conducting surfaces. A wrist carried selective call receiver thus must be designed for acceptable receive sensitivity in many possible body positions.
Conventional wrist carried receivers have used serpentine loop antennae or simple loop antennae that are responsive to the magnetic field component of a transmitted radio frequency (RF) signal. One such antenna system is shown in U.S. Pat. No. 3,032,651 to Gisiger-Stahli et al. This type of antenna system provided marginal performance but was desirable because the antenna elements could be concealed within the wrist band housing. The performance of simple loop antennae was particularly poor for certain body positions in which the magnetic field of the transmitted RF signal was parallel to the plane of the loop.
An improvement to the single loop antenna design is shown in U.S. Pat. No. 4,873,527 to Tan. The improvement comprises a "combo" design incorporating a ferrite core loop antenna connected in series with a single magnetic loop antenna. The axes of the two antennae were orthogonal to one another, resulting in a reduction in susceptibility to performance variation caused by changes in body position. While the "combo" design performed better than the single loop design, there were size and weight tradeoffs that limited its performance. Because the ferrite core loop antenna had to fit within the housing of the selective call receiver, it had to be small and light. A small ferrite core antenna, though, would be less sensitive than one with a larger cross-sectional area. The result invariably was either a selective call receiver that was larger than desired, or a design whose susceptibility to performance variation caused by changes in body position was more than desired.
A further problem with the "combo" design resulted from the fact that the ferrite core loop antenna and the single magnetic loop antenna were connected in series. The single magnetic loop antenna consisted of two elements that became separated whenever the wristband clasp was opened, e.g., to remove the wrist carried selective call receiver. Opening the clasp would thus also open the entire antenna system, substantially reducing its performance. Reduced performance with the clasp open is undesirable in the wrist carried selective call receiver, because some users might want to continue operating the selective call receiver while not actually wearing it about the wrist.
What therefore is needed is an antenna system for a wrist carried selective call receiver that eliminates the space consuming ferrite core loop antenna while maintaining antenna performance in many possible body positions. An antenna system that continues to function when the selective call receiver is removed from the wrist with the clasp open also is desirable.